This invention relates to a process for reducing the sulfur content of coal.
It is recognized that an air pollution problem exists whenever sulfur-containing fuels are burned. The resulting sulfur oxides are particularly objectionable pollutants because they can combine with moisture to form corrosive acidic compositions which can be harmful and/or toxic to living organisms in very low concentrations.
Coal is an important fuel and large amounts are burned in thermal generating plants primarily for conversion into electrical energy. Many coals generate significant and unacceptable amounts of sulfur oxides on burning. The extent of the air pollution problem arising therefrom is readily appreciated when it is recognized that coal combustion currently accounts for 60 to 65% of the total sulfur oxides emissions in the United States.
The sulfur content of coal, nearly all of which is emitted as sulfur oxides during combustion, is present in both inorganic and organic forms. The inorganic sulfur compounds are mainly iron pyrites, with lesser amounts of other metal pyrites and metal sulfates. The organic sulfur may be in the form of thiols, disulfides, sulfides and/or thiophenes chemically associated with the coal structure itself. Depending on the particular coal, the sulfur content may be primarily either inorganic or organic. Distribution between the two forms varies widely among various coals. For example, both Appalachian and Eastern interior coals are known to be rich in both pyritic and organic sulfur. Generally, the pyritic sulfur represents from about 25% to 70% of the total sulfur content in these coals.
Heretofore, it has been recognized to be highly desirable to reduce the sulfur content of coal prior to combustion. In this regard, a number of processes have been suggested for physically reducing the inorganic portion of the sulfur in coal. Organic sulfur cannot be physically removed from coal.
As an example, it is known that at least some pyritic sulfur can be physically removed from coal by grinding and subjecting the ground coal to froth flotation or washing processes. These processes are not fully satisfactory because a significant portion of the pyritic sulfur and ash are not removed. Attempts to increase the portion of pyritic sulfur removed have not been successful because these processes are not sufficiently selective. Because the processes are not sufficiently selective, attempts to increase pyrite removal can result in a large portion of coal being discarded along with ash and pyrite.
There have also been suggestions heretofore to remove pyritic sulfur from coal by chemical means. For example, U.S. Pat. No. 3,768,988 discloses a process for reducing the pyritic sulfur content of coal by exposing coal particles to a solution of ferric chloride. The patent suggests that in this process ferric chloride reacts with pyritic sulfur to provide free sulfur according to the following reaction process: EQU 2FeCl.sub.3 +FeS.sub.2 .fwdarw.3FeCl.sub.2 +2S.
While this process is of interest for removing pyritic sulfur, a disadvantage of the process is that the liberated sulfur solids must then be separated from the coal solids. Processes involving froth flotation, vaporization and solvent extraction are proposed to separate the sulfur solids. All of these proposals, however, inherently represent a second discrete process step, with its attendant problems and cost, to remove the sulfur from coal. In addition, this process is notably deficient in that it does not remove organic sulfur from coal.
In another approach, U.S. Pat. No. 3,824,084 discloses a process involving grinding coal containing pyritic sulfur in the presence of water to form a slurry, and then heating the slurry under pressure in the presence of oxygen. The patent discloses that under these conditions the pyritic sulfur (for example, FeS.sub.2) can react to form ferrous sulfate and sulfuric acid which can further react to form ferric sulfate. The patent discloses that typical reaction equations for the process at the conditions specified are as follows: EQU FeS.sub.2 +H.sub.2 O+2 O.sub.2 .fwdarw.FeSO.sub.4 +H.sub.2 SO.sub.4 EQU 2FeSO.sub.4 +H.sub.2 SO.sub.4 +1/2 O.sub.2 .fwdarw.Fe.sub.2 (SO.sub.4).sub.3 +H.sub.2 O.
Accordingly, the pyritic sulfur content continues to be associated with the iron as sulfate. Several factors detract from the desirability of this process. High temperatures and pressures are employed which can necessitate the use of expensive reaction vessels and processing plants of complex mechanical design. Because high temperatures are employed, excessive amounts of energy can be expended in the process. In addition, the above oxidation process is not highly selective in that considerable amounts of coal itself are oxidized. This is undesirable, of course, since the amount and/or heating value of the coal recovered from the process is decreased.
Heretofore, it has been known that coal particles could be agglomerated with hydrocarbon oils. For example, U.S. Pat. Nos. 3,856,668 and 3,665,066 disclose processes for recovering coal fines by agglomerating the fine coal particles with oil. U.S. Pat. Nos. 3,268,071 and 4,033,729 disclose processes involving agglomerating coal particles with oil in order to provide a separation of coal from ash. While these processes can provide some benefication of coal, better removal of ash and iron pyrite mineral matter would be desirable.
The above U.S. Pat. No. 3,268,071 discloses the successive removal of two particulate solid minerals or metals having respectively hydrophilic and hydrophobic surfaces relative to the suspending liquid phase, by staged agglomeration with addition in each stage of a separate bridging liquid capable of preferentially wetting respectively the hydrophilic or the hydrophobic surfaces.
The above U.S. Pat. No. 4,033,729 relating to removing inorganic materials (ash) from coal significantly notes that iron pyrite mineral matter has proven difficult to remove because of its apparent hydrophobic character. This disclosure confirms a long-standing problem. The article, "The Use of Oil in Cleaning Coal", Chemical and Metallurgical Engineering, Volume 25, pages 182-188 (1921), discusses in detail cleaning coal by separating ash from coal in a process involving agitating coal-oil-water mixtures, but notes that iron pyrite is not readily removed in such a process.
In a process effecting agglomeration of coal particles, as by contacting with a suitable quantity of oil in an aqueous medium, the physical dimensions of the coal particles are altered. The larger coal agglomerates may suitably be separated from the slurry systems by passage over screens or sieves to retain the enlarged coal particles while permitting passage of unincorporated or unattached mineral matter which retains its original particle size in the aqueous slurry.
Froth flotation techniques have been used for some time, particularly in Europe, for recovery of fine coal. In effect, air bubbles are formed and the solid coal surfaces become attached to the bubbles with the aid of collectors. The most efficient air-solid interfaces form with hydrophobic solids such as coal.
Dissolved gas flotation techniques (as distinguished from dispersed gas flotation) have been used for removing coal and pyrite from slate, clay and other contaminants. A suitable inert gas (air, carbon dioxide, light hydrocarbon) dissolved, for example, in water under pressure will, when pressure is reduced, be liberated in very fine bubbles. Such small bubbles are especially effective for solid surfaces attachment, particularly hydrophobic surfaces such as exhibited by coal.
Some recent attention has been given to possible application of the Reichert cone concentrator, a high-capacity wet gravity concentration device developed in Australia, to the removal of ash and inorganic sulfur from coal. It is used commercially for gravity concentration of mineral sands.
Recent studies have also been conducted by the U.S. Bureau of Mines on physical desulfurization of fine-size coals employing the Humphreys spiral concentrator, a mineral-dressing device not heretofore accepted in the coal industry. (Bureau of Mines Report RI-8152/1976).
Other techniques employing density differentials have similarly been considered, as, for example, hydroclones and centrifugal whirlpool arrangements.
While there is much prior art relating to processes for removing sulfur and ash from coal, there remains a pressing need for a simple, efficient process for removing sulfur and ash from coal. Such a process must maximize recovery of the carbon heating value of the coal as well as reduction of the ash and sulfur content.